A number of organic compounds which have the flow characteristics of liquids and yet which show crystalline properties have been known for about a century. These materials, known as liquid crystals, have recently received intensive investigation because of their usefulness in display devices operable at extremely low energy levels. In such display devices the physical nature and especially, the optical properties of the material can be changed by the application of electric or magnetic fields of sufficient strength.
Liquid crystals are classified roughly into three types, namely smectic, cholesteric and nematic. It is the nematic type of liquid crystal which has proved to be most useful for display devices; nematic crystals are divided into two classes, these classes differing somewhat in their optical properties and the differences being the basis for two different types of display systems. These are as follows:
1. A display in which a small current flows, the ions disturbing the crystallinity of the liquid crystal material and causing scattering of light. Although non-destructive over moderate periods of time, eventually, the current flowing through the system gives rise to oxidative or reductive degradation of the liquid crystal material and the electrodes.
2. If the liquid crystal molecules are placed in preferred orientations, the material is birefringent. Using proper techniques, the material, when placed between opposing plates, can be caused to rotate the plane of polarized light through a desired angle so that when used in combination with a pair of polarizing plates, the system can act as an optical shutter. To act as an optical shutter, the optical activity of the liquid crystal material must be eliminated. This can be done by imposing an electric field across the liquid crystal material.
The electro-optical properties of the nematic liquid crystals depend upon whether their dielectric anisotropy is positive or negative. Those used in the first type of display described above have negative dielectric anisotropy, whereas the liquid crystal materials or compositions utilized in the second type of display have positive dielectric anisotropy.
In nematic liquid crystals having positive dielectric anisotropy, a dipole is present which is essentially parallel with the principal axis of the molecule; when placed in an electric field of sufficient strength, the dipole aligns itself with the electric field, and consequently, the principal or macro-axis of the molecule likewise aligns itself with the electric field. In contrast, where the liquid crystal material is one whose dielectric anisotropy is negative, the direction of the electric dipole is essentially at right angles to the direction of the macro-axis of the molecule. Consequently, in the presence of an electric field the macro-axis will lie in a plane perpendicular to the electric field direction so that the orientation of the molecule is indeterminate.
To utilize a nematic liquid crystal material or composition having a net positive dielectric anisotropy, the liquid crystal is sealed between two transparent plates, on the inner surface of each of which are one or more transparent electrodes in the form of a mosaic or numerical segments. Prior to placing the plates in opposition, the surfaces are rubbed with gauze or absorbent cotton or the like, forming, it is believed, minute grooves. The rubbing is carried out in a single direction so that the grooves on a single plate are all parallel to each other. In mounting the plates together, they are set so that the rubbing directions on the two plates are at right angles to each other. The liquid crystal molecules immediately adjacent to each of the plates align themselves with the rubbing direction, that is, with the grooves. The liquid crystal molecules between the plates orient themselves into a helix the ends of which correspond with the rubbing directions on the plates. When the plates are at right angles to each other, the helix makes one-quarter turn, as a result of which linearly-polarized light traversing the cell is rotated through an angle of 90.degree.. In the usual construction, such a cell is sandwiched between upper and lower polarizing plates or filters. Assuming the axes of the polarizing plates to be at right angles to each other, incident light will be transmitted through the cell due to the optical activity of the liquid crystal material between the plates of the cell. This assumes, of course, that no electric field is imposed across the cell.
When the axes of the polarizing plates are parallel to each other, no light will traverse the cell. However, if an electric field is applied to selected transparent electrodes on the upper and lower plates of the liquid crystal cell, as aforenoted, the liquid crystal composite will lose its optical activity because the macro-axis of the molecule will align itself parallel to the applied electric field. Now, assuming the polarizing filters to be placed at right angles to each other, as the optical activity is lost from those portions of the cell across which the electric field is imposed, those portions will become opaque while the remainder of the cell remains transparent. If the polarizing filters are parallel, the converse will take place.
Rubbing of the electrode surfaces prior to assembly of the cell plates into a cell with gauze, absorbent cotton or the like is believed to produce minute grooves in the surface of the cell plates. The liquid crystal molecules adjacent to the surface of the rubbed plate align themselves with the groove, that is, parallel to the rubbing direction. However, conventional rubbing techniques wherein the surfaces of the transparent electrodes as well as the surfaces of the glass plates are rubbed do not necessarily give uniform alignment over the entire exposed surface, due largely to the fact that impurities are present on the surface of the transparent electrodes and of the glass plates themselves. Even if the surfaces are cleaned very thoroughly prior to the rubbing operation, the alignment is imperfect. Accordingly, in production of such display cells the yield of satisfactory cells is limited and costs, consequently, are relatively high. Moreover, as the cell ages the liquid crystal material deteriorates in quality so that the light transmission therethrough as well as the contrast yielded by the display is impaired. These are the problems which the present invention is designed to overcome.